Use of high resolution 3D seismic data to evaluate Quaternary valley evolution history during transgression, offshore San Luis Pass, Gulf of Mexico



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A novel, shallow-investigation, high-resolution 3D (HR3D) seismic acquisition system has been employed, for the first time in the Gulf of Mexico, to characterize CO₂ storage potential and de-risk targets for sequestration. HR3D data can image detailed depositional, architectural, and structural features in the shallow subsurface that have previously been below seismic resolution and/or excluded from industry surveys, which are optimized for deeper targets. One HR3D survey was collected in 2013 offshore San Luis Pass, TX and covers an area of 31.5 km². The dataset images the upper 500 meters of stratigraphy with unprecedented detail -- peak frequency of approximately 150Hz (eight 25m cables, spaced at 12.5m, 6.25m by 6.25m bin size). Imaged within this dataset at ~100ms TWTT, is a mappable erosional unconformity that is interpreted to be associated with the Brazos River system during the ~130ka glacial-eustatic lowstand and following transgression. Through the analysis of horizon slices and the geometries of the valley form and its dendritic features, the evolution history of the valley system during a transgressive episode can be characterized. Observations indicate that the system evolves from a lowstand meandering channel system with clear point-bar deposits to a transgressive estuary characterized by dendritic erosional features that is eventually flooded. These 3D data represent an exceptional example of a lowstand to transgressive transition and the sedimentary processes and architectures that characterize each interval. A seismically discontinuous zone is observed within the HR3D volume that is interpreted to be a gas chimney system emanating from a tested dry, 3-way structure in the lower Miocene (1.5km depth). Within the shallowest intervals (<100m) and at the top of the chimney zone, seismic attribute analysis reveals several high amplitude anomalies that are predominantly located within interpreted interfluvial zones. The anomalies fit into our stratigraphic and structural interpretation of the interval, in that they appear to sit at local structural, fault bounded highs within deposits interpreted to be coarser grained and are overlain by finer grained, transgressive deposits. These observations support the interpretation of these amplitude anomalies as shallow gas accumulations derived from a deeper, depleted gas reservoir. Interestingly, point-bar deposits as well as channel scour deposits within the same interval show no sign of charge, suggesting that these are either isolated from the migration flowpath, or too fine-grained to host significant saturations.